It is well-known in the ocular glass field to use inorganic colloids in scratch-resistant and/or abrasion-resistant coatings (also referred to as “hard coats”) for improving the surface properties of organic glasses and/or for modifying the refractive index thereof.
Zirconia, or zirconium dioxide, is one of these known colloids, because it provides a number of useful mechanical and physical properties, including high fracture toughness, mechanical strength and hardness, low thermal conductivity, high refractive index and transparency in the visible and infra-red spectral regions.
However, it may be difficult to produce in a simple and reproducible manner a colloidal suspension of zirconia with high crystallinity in the form of a stable homogeneous dispersion of nanoparticles (having an average particle size of less than 20 nm) at high solids content (at least 20 wt. % and up to 35 wt. %), and also to avoid their aggregation both during the preparation of the sol and during incorporation into a polymer matrix, for instance an epoxysilane matrix. Such a homogeneous dispersion is however required for the formation of a transparent zirconia-containing composite film and it directly affects the haze and transparency of the optical coating. This stable homogeneous dispersion may be expressed by the zeta potential of the suspension, which absolute value should be of at least 30 mV, and by its viscosity, which should be less than 10 cPs at 25° C. (no gelling should occur). The transparency is also ensured by the low particle size with uniform and narrow size distribution.
Thus, the methods that have been proposed to prepare colloidal zirconia, result either in a product having a low crystallinity, which affects the refractive index of zirconia colloids, and also their application in optical coatings (EP 0 229 657), or in dry powders made of particles which, when redispersed in a solvent, will remain aggregated in some extent (CN-101613120; by HUANG Y. et al. in Wujiyan Gongye, 37(7), 15-17 (2005). This will also be detrimental to the transparency of the colloidal suspensions obtained. These colloids are not suitable for preparing optical coatings.
Moreover, US 2010/0144918 and WO 2008/139100 disclose a method for preparing colloidal zirconia suspensions which may be used in the manufacture of hard-coats. This method in several synthesis steps results in an acidic zirconia sol in which zirconia is provided as crystals having a mainly tetragonal crystalline phase and which are suspended in an aqueous medium. These tetragonal crystals cannot be easily dispersed in the silane-based sol-gel hard-coat composition, neither when the crystals are dispersed in water, nor when they are dispersed in methanol after a solvent-exchange step. The appearance of the resulting hard-coat composition thus becomes progressively milky or colloid precipitation occurs, which is not desirable for the final transparency of the coating.
Other acidic zirconia sols comprising zirconia particles having mainly tetragonal and cubic crystal lattice structures have been disclosed in US 2002/004544. These crystals are considered as providing a lower aspect ratio than those having a monoclinic crystal phase, which is said to be favourable to their incorporation in high amounts in organic matrixes.
Other zirconia colloids are currently being sold for the manufacture of hard coats, such as those marketed by NISSAN CHEMICAL. The nano-particles in these colloids comprise other metal oxides besides zirconia.
The inventors have now discovered that zirconia sols comprising particles which have mainly a monoclinic crystal lattice structure allowed the formation of a composition with high solids content, which could be used to form a transparent hard-coat under economical conditions. Surprisingly, it has also been found that this coating could be applied directly to the polymer substrates generally used in ophthalmic lenses, without the need for any physical pre-treatment of the substrate like plasma, corona or UV irradiation, or inserting a primer coating (such as a polyurethane latex or an aminosilane layer) between the substrate and the hard-coat in order to improve the adhesion of the hard-coat. Moreover, the inventors have shown that this holds true also for acrylic substrates which are known to provide poor adhesion to their coatings absent the above treatments. This invention thus offers a way to produce low-cost ophthalmic lenses with high production yield, because it does neither require expensive treatments of the substrate nor the provision of a primer.